Calibrating the Counter part 1 Test Equipment (George Dowell)
- Most all meters (counters) have a calibration pot (i.e. CDV700), some have
one for each range.
The basic function of all counters can be broken down into these simple
A) Create a stable high voltage DC that supplies the probe through a load
B) Decouple the resulting pulse from that DC and process it in an amplifier
C) Take the output of the amplifier and count the pulses for display on a
D) Further provide for an audio output representation of the pulses
Vastly different approaches are used to accomplish some or all of the above,
depending on manufacturer, and price bracket within the product line.
The task of properly calibrating a counter would take some pretty expensive
instrumentation, certainly out of reach of a duffer like myself. For our
purposes though some surprisingly inexpensive test gear will give adequate
My test bench for counters includes an oscilloscope, a high range Ohmmeter,
a kilovolt meter, and a pulse generator. Total cost : less than $50.00
The central meter that performs all of the measuring is an old RCA Senior
VoltOhmyst, model WV-98C. I was lucky enough to find what appeared to be a
brand new unit at a hamfest, with manual for $10.00. These can be had for
peanuts from eBay, because no one wants an analog readout tube instrument
any more. By teaming up the basic meter with some outboard accessories, a
wide range of high voltages ( at high impedance load) and high resistances
can be measures with fair accuracy (+/- 3%).
A very wide range of AD/DC Voltages, and resistance can be measured with the
basic instrument ( sorry, no current, use a Simpson 260 for this).
Measuring high voltage produced by a counter requires a very high impedance
instrument be used, namely 1000 MegOhms or more. The VoltOhmyst's input
impedance is 11 MegOhms, far to low by itself, but they make an accessory
100:1 divider probe that increases this to 1100 MegOhms ( another $10.00
purchase. Look for model WG-411A or WG-289....TV shop items). Perfect.
Measuring high resistance resistors and capacitor leakage can be
accomplished directly by the meter up to 1000 MegOhms. This represents a
huge value, but some "Geiger counters" are actually ion
chamber/electrometers and really large values of resistors are encountered.
Additionally most small capacitor's leakage will exceed this value. ( note:
much better not to muck around in the "high impedance" areas of any meter.
Even a fingerprint can compromise the circuitry) Anyhow in order to measure
those extremely high resistances, all you need to do is put the device under
test in series with an external power supply of up to 500 Volts, then put
the meter in the Voltage mode and read the resulting small voltage. A little
math** will reveal the actual resistance of the test item, knowing that the
VoltOhyst itself is equivalent to an 11 MegOhm resistor. A simple voltage
divider is produced by the 11 meg meter resistance and the test items actual
resistance, and the voltage at the node is the reading on the meter, which
is compared to the total voltage of the power supply.
Rx = 11 [ (Volts at power supply) - (Volts at node)] / Volts at node
Where Rx is the test item resistance in MegOhms.
The oscilloscope can be another $10.00 item for no great high frequency
response is needed to look at the waveforms that exist in a Geiger counter.
It is an invaluable tool for tracing the path of the pulses though, and for
guestimating stage gain. Measures the pulse right from the probe through to
Now the "pulse generator" can be nothing more complicated than an audio
oscillator with a square wave output. Nothing magic about this, and the one
I have is not even continuously adjustable, It's an RSR Electronics SG-105
RC Oscillator, and has 8V p-p output.Cost about $20.00 new and runs on a 9
volt battery. Has 23 steps with a x1 and x 10 switch. Any real function
generator will do the job as well or better, but at a cost.
All you are doing is injecting a known pulse into the amplifier to simulate
the pulses that come from the probe, but this way you know the exact
frequency of the pulses ahead of time.
Just figure out the CPM by multiplying CPM by 60 to get CPS or Hertz. Find a
known frequency that will give you about 80% full scale reading and inject
it into the counter on the low side ( not the probe side, high voltages
exist there) of the probe decoupling capacitor. Insert an outboard .47 mFd
cap in series with the test lead to keep the generator for encountering any
low DC voltages. Set the cal pot for the correct reading and then select
another tone frequency that will yield about a 20% full scale deflection.
This checks that range for linearity. Not much can be done if it's off a
little, but at least you'll know what's going on. Do the same on each range
if there are adjustments for each, otherwise go through the exercise to
verify the proper operation of the ranging components.
Going through all your various meter in this fashion will assure that they
are all about the same, and then probes can freely be swapped between them
with a reasonable feeling of compatibility.